Microelectronic applications of Group III-V nitride semiconductor materials have recently been investigated. Group III-V nitride semiconductor materials include aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) and their related ternary and quaternary alloys such as aluminum gallium nitride (AlGaN).
Group III-V nitride semiconductors have bandgaps ranging from 1.9 eV to 6.2 eV as shown in FIG. 1. Thus, these semiconductor materials are suitable for a range of potential applications including ultraviolet to visible optoelectronics (for example LEDs and lasers) and high temperature electronics (for example transistors). In addition, the negative electron affinity (NEA) nature of the conduction band of AlN makes this Group III-V nitride semiconductor a potential new and efficient electron source in cold cathode, microelectronics, and flat panel electro-luminescent display applications. See, for example, review articles by Strite et al. entitled GaN, AlN, and InN: A Review, Journal of Vacuum Science and Technology B, Vol. 10, pp. 1237-1266, 1992, and Morkoc et al. entitled Large-Band-Gap SiC, III-V Nitride, and II-VI ZnSe-Based Semiconductor Device Technologies, Journal of Applied Physics, Vol. 76, pp. 1363-1398, 1994.
Recent advances in Group III-V nitride device development include the demonstration of high-brightness blue light-emitting diodes as described in the publication by Nakamura et al. entitled Candela-Class High-Brightness InGaN/AlGaN Double-Heterostructure Blue-Light-Emitting Diodes, Applied Physics Letters, Vol. 64, pp. 1687-1689, 1994. A second group of researchers has demonstrated transistor structures based on Group III-V nitrides as described in the publications by Khan et al. entitled Metal Semiconductor Field Effect Transistor Based on Single Crystal GaN, Applied Physics Letters, Vol. 62, pp. 1786-1787, 1993 and High Electron Mobility Transistor Based on a GaN-Al.sub.x Ga.sub.1-x N Heterojunction, Applied Physics Letters, Vol. 63, pp. 1214-1215, 1993.
Several groups of researchers also report optically-pumped stimulated emission from III-V nitride structures, which can form the basis for laser diodes. See the publications by Amano et al. entitled Room-Temperature Violet Stimulated Emission from Optically Pumped AnGaN/GaInN Double Heterostructure, Applied Physics Letters, Vol. 64, pp. 1377-1379, 1994, and Yung et al. entitled Observation of Stimulated Emission in the Near Ultraviolet from a Molecular Beam Epitaxy Grown GaN film on Sapphire in a Vertical-Cavity, Single Pass Configuration, Applied Physics Letters, Vol. 64, pp. 1135-1137, 1994.
Accordingly, Group III-V nitride compound semiconductors are expected to play an increasingly important role in high-temperature microelectronics. Unfortunately, there are presently two fundamental obstacles to the design and fabrication of Group III-V nitride compound semiconductor devices: the lack of a suitable lattice-matched and conducting substrate, and the lack of a suitable ohmic contact for these materials. Each of these fundamental obstacles will now be described.
The first fundamental obstacle which presently limits the overall quality of Group III-V nitride films and devices is the lack of a suitable lattice-matched and preferably conducting substrate for Group III-V nitride growth. Bulk substrates of single-crystal Group III-V nitrides are not presently available. As a consequence, sapphire and silicon carbide (SiC)--both of which have lattice constants that are appreciably different from those of the III-V nitrides as listed in Table I--are currently preferred substrates for Group III-V nitride film growth.
For growth on sapphire, which is an electrically insulating substrate material, a two-step growth process has been employed for growth of GaN-based materials. Amano et al. in the publication entitled Metalorganic Vapor Phase Epitaxial Growth of a High Quality GaN Film Using an AlN Buffer Layer, Applied Physics Letters, Vol. 48, pp. 353-355, 1986, describes the use of a thin buffer layer of AlN grown at low temperatures (about 600.degree. C.) on sapphire. The temperature is then raised to about 900.degree.-1100.degree. C. for growth of GaN. In U.S. Pat. No. 5,290,393, Nakamura describes the use of a Al.sub.1-x Ga.sub.x N buffer layer (0.ltoreq.x.ltoreq.1) grown at low temperatures (400.degree.-800.degree. C.) on sapphire followed by growth of GaN at a higher temperature (about 900.degree.-1000.degree. C.). More specifically, Nakamura et al. in the publication entitled Candela-Class HighBrightness InGaN/AlGaN Double-Heterostructure Blue-Light-Emitting Diodes, Applied Physics Letters, Vol. 64, pp. 1687-1689, 1994, employs a 300 .ANG. thick GaN buffer layer grown at 510.degree. C. on sapphire. Next, the substrate temperature is elevated to 1020.degree. C. to grow GaN films. Similar processes have also been employed for growth of III-V nitride films on SiC. However, it has been generally found that crack-free III-V nitride growth on SiC requires the use of an AlN buffer layer. Buffer layers of GaN or Al.sub.1-x Ga.sub.x N often result in III-V nitride film growth which contain networks of cracks. This is unacceptable for device applications.
TABLE I ______________________________________ PROPERTIES OF SELECTED SEMICONDUCTORS Lattice Band Material Constant gap (eV) Thermal Expansion ( .times. 10.sup.-6 /.degree.K.) ______________________________________ GaN a = 3.189 .ANG. 3.39 (300K) .DELTA.a/a = 5.6 (300-900.degree. K.) c = 5.185 .ANG. 3.50 (1.6K) .DELTA.c/c = 3.2 (300-700.degree. K.) .DELTA.c/c = 7.8 (700-900.degree. K.) AIN a = 3.112 .ANG. 6.2 (300K) .DELTA.a/a = 5.3 (300-1100.degree. K.) c = 4.982 .ANG. 6.28 (5K) .DELTA.c/c = 4.2 (300-1100.degree. K.) InN a = 3.548 .ANG. 1.89 (300K) .DELTA.a/a = 3.8-6.0 (300-600.degree. K.) c = 5.760 .ANG. .DELTA.c/c = 3.0-3.8 (300-600.degree. K.) Sapphire a = 4.758 .ANG. .DELTA.a/a = 7.3-7.7 (300-1100.degree. K.) c = 12.991 .ANG. .DELTA.c/c = 8.1-8.6 (300-1100.degree. K.) SiC (6H) a = 3.08 .ANG. 2.86 (300K) .DELTA.a/a = 4.2-5.4 (700-1500.degre e. K.) c = 15.12 .ANG. .DELTA.c/c = 4.7-4.9 (700-1500.degree. K.) ZnO a = 3.252 .ANG. 3.30 (300K) .DELTA.a/a = 4.8-6.0 (300-400.degree. K.) c = 5.213 .ANG. .DELTA.a/a = 7.2-8.3 (500-800.degree. K.) .DELTA.c/c = 2.9-3.8 (300-400.degree. K.) .DELTA.c/c = 4.4-5.0 (500-800.degree. K.) Si a = 5.4301 .ANG. 1.10 (300K) .DELTA.a/a = 3.2-5.6 (300-1100.degree. K.) GaAs a = 5.6533 .ANG. 1.43 (300K) .DELTA.a/a = 5.0-6.1 (200-600.degree. K.) ______________________________________
The use of a low-temperature buffer layer on sapphire or SiC has allowed Group III-V nitride films to be fabricated. Unfortunately, the two-temperature technique has not heretofore been able to produce nitride layers having sufficiently low dislocation density, to the best of the present inventor's knowledge for many potential device applications. It is generally known that Group III-V nitride materials grown on sapphire or SiC substrates contain 10.sup.9 -10.sup.11 dislocations per cm.sup.2. By comparison, Group II-VI semiconductor devices based on ZnSe or related alloys generally contain less than 10.sup.6 dislocations per cm.sup.2, and Group III-V As-based and P-based semiconductor devices contain less than 10.sup.4 dislocations per cm.sup.2.
In addition, the large difference in thermal expansion coefficients between SiC and GaN presents problems. Since the expansion coefficient (.DELTA.a/a) of SiC is less than that of GaN (see Table I above) upon cooling to room temperature after thin film growth, the GaN film on SiC is under tension. As is well-known to those skilled in the art of semiconductor film growth, this in itself can lead to cracking effects which destroy the overall quality of the epitaxial layer.
However, it is extremely desirable that Group III-V nitride materials be grown on a conducting substrate, particularly for device applications involving vertical transport of carriers. Such devices include light-emitting diodes, laser diodes, and certain transistor structures, for example. The Nakamura et al. blue LED discussed above requires non-standard processing and packaging techniques because the sapphire substrate is electrically insulating. Specifically, as described by Nakamura et al. in Candela-Class High-Brightness InGaN/AlGaN Double-Heterostructure Blue-Light-Emitting Diodes, Applied Physics Letters, Vol. 64, pp. 1687-1689, 1994, InGaN/AlGaN mesa LED structures must be fabricated using photolithographic and etching techniques so that both the metallic electrical contact to the top p-type layer of the device and the base metallic electrical contact to the bottom n-type layer of the device can be made from the top surface of the wafer using wire bonding techniques. This approach is required due to the insulating nature of the sapphire substrate which is used.
A conducting substrate such as SiC is much preferred since a conducting substrate allows the LED base metallic electrode to be located on the bottom surface of the substrate, rather than at the top surface. As a consequence, packaged LED lamps can be fabricated more efficiently using standard techniques which employ silver epoxy to secure the LED base electrode and require only one wire-bonded top contact--an important cost-saving advantage in an LED production facility. In addition, vertical transport through a low-resistance conducting substrate such as SiC may be essential for the future development of other optoelectronic devices based on III-V nitride semiconductors such as laser diodes. It is well known to those skilled in the art that a laser diode requires a much higher current density when operating above threshold than does an LED. As a consequence, the series resistance of the device must generally be as small as possible to minimize heating effects which can lead to premature device degradation and failure. This generally requires the use of a conducting substrate. Accordingly, there is a need for a conducting substrate for Group III-V nitride semiconductor materials.
The ohmic contact problem for Group III-V nitride semiconductors will now be described. Important advances in understanding the fundamental properties of Group III-V nitride materials have recently been made by several research groups. Benjamin et al., in the publication entitled Observation of a Negative Electron Affinity for Heteroepitaxial AlN on .alpha.(6H)-SiC(0001), Applied Physics Letters, Vol. 64, pp. 3288-3290, 1994, report convincing evidence based on ultraviolet photoemission spectroscopy (UPS) that AlN is a negative electron affinity (NEA) material. In other words, the conduction band of AlN lies above the vacuum energy level implying that AlN can be used as an efficient emitter of electrons. Consistent with these findings, the above investigators also report the valence band offset between AlN(0001) and SiC(0001) to be approximately 0.8 eV.
Three other research groups have recently reported values for the valence band offset between AlN and GaN. Martin et al. in the publication entitled Valence-Band Discontinuity Between GaN and AlN Measured by X-Ray Photoemission Spectroscopy, Applied Physics Letters, Vol. 65, pp. 610-612, 1994, report a Type I heterojunction (valence band edge of AlN below that of GaN) with a valence band offset or discontinuity of .DELTA.E.sub.v =0.8.+-.0.3 eV. Baur et al. in the publication entitled Determination of the GaN/AlN Band Offset Via the (-/0) Acceptor Level of Iron, Applied Physics Letters, Vol. 65, pp. 2211-2213, 1994, report a Type 1 heterojunction with a valence band discontinuity of .DELTA.EV=0.5 eV. Segall et al., in a paper presented at the 2nd Workshop on Wide Bandgap Nitrides held Oct. 17-18, 1994 in St. Louis, Mo. entitled Band-Offsets and Related Properties of III-N's, report .DELTA.E.sub.v =0.8 eV for the valence band offset between AlN and GaN. In addition, these researchers report a Type 1 interface between GaN and InN with .DELTA.E.sub.v =0.5 eV. Segall et al. also report a Type 1 interface between AlN and GaAs with .DELTA.E.sub.v =2.0 eV.
The above results for band offsets have important consequences concerning the transport of electrons and holes through interfaces involving Group III-V nitride materials. FIG. 2 summarizes these results by illustrating schematically, in terms of energy band diagrams, how the conduction and valence bands of the binary Group III-V nitride semiconductors line up relative to one another and to other well-known semiconductor materials GaAs, Si and SiC. It will be recognized by those skilled in the art of semiconductor devices that FIG. 2 lists approximate band offsets among the various materials that are shown, based upon the above described reports. These band offsets may only be accurate to within .+-.0.2-0.3 eV, based on the accuracy of current experimental measurement techniques.
As is known by those skilled in the art, heterojunction energy barriers in excess of about 0.3 eV can prevent the flow of carriers (electrons and/or holes) in thin film devices which require vertical transport of charged carriers across heterointerfaces. Devices of this type include light emitting diodes, laser diodes, certain transistor structures, and electron emitters based on NEA materials such as AlN, for example. The band diagram of FIG. 2 clearly shows that there can be substantial energy barriers when these types of devices are based on III-V nitride heterostructures. Accordingly, contacts to Group III-V nitride compound semiconductor materials, using conventional metals such as silver and gold, are not ohmic.
The ohmic contact problem for Group III-V nitride compound semiconductors has recently been recognized by those skilled in the art other than the present inventor. See for example, the publication by Foresi and Moustakas at Boston University entitled Metal Contacts to Gallium Arsenide, Applied Physics Letters, Vol. 62, No. 22, pp. 2859-2861, May 1993, which reports an initial investigation of aluminum and gold contacts to gallium nitride. Both aluminum and gold contacts are reported as being ohmic. However, the contact resistivity of the aluminum and gold contacts were found to be 10.sup.-7 -10.sup.-8 -m.sup.2. These contact resistances are several orders of magnitude greater than is generally required for laser diodes. A more recent publication by Molnar, Singh and Moustakas at Boston University, entitled Blue-Violet Light Emitting Gallium Nitride p-n Junctions Grown by Electron Cyclotron Resonance-Assisted Molecular Beam Epitaxy, Applied Physics Letters, Vol. 66, No. 3, Jan. 16, 1995, notes that ohmic metal contacts to p-type gallium nitride would require a metal with a work function close to 7.5 eV. The Molnar, Singh and Moustakas paper notes that such a metal is not available. This paper then reports on the use of Ni/Au to contact p-type GaN layers and In to contact n-type GaN layers. The resulting current-voltage characteristics as measured and reported are very poor. Accordingly, while those skilled in the art of Group III-V nitride compound semiconductors have recently recognized the lack of a suitable ohmic contact, a solution to this problem has not, to the best of the inventor's knowledge, been found.
In order to provide an ohmic contact to common intermetallic semiconductors such as GaAs, Woodall described in U.S. Pat. No. 4,801,984 the use of Group III-V ternary graded layers of InGaAs to make good electrical contact to GaAs. More recently, the present inventor described in U.S. Pat. Nos. 5,294,833, 5,351,255, and 5,366,927, ohmic contacts to Group II-VI materials using, for example, graded layers of ZnHgSe or ZnTeSe to make ohmic contact to Group II-VI blue/green light emitting devices.
However, it will be recognized by those skilled in the art that neither of the above contact systems can be used for Group III-V nitride materials, since the Group III-V nitride semiconductors have a hexagonal crystal structure which is incompatible with the cubic crystal structure of the Group III-V arsenides/phosphides and the Group II-VI materials based on ZnSe and related alloys. In addition, the basal plane lattice constants of the Group III-V nitrides are substantially different from the lattice constants and (111)-plane nearest-neighbor-distances of the Group III-V arsenides/phosphides and the Group II-VI materials based on ZnSe and related alloys. See FIG. 1.
The above survey indicates that, although significant advances have recently been made in demonstrating Group III-V nitride devices, a number of problems remain to be addressed. Specifically, Group III-V nitride materials grown to date have very high dislocation densities (.gtoreq.10.sup.9 per cm.sup.2) due to the unavailability of lattice-matched bulk nitride substrates. In addition, the use of nonconductive substrates such as sapphire presently limit the use of Group III-V nitride materials to device applications which do not require vertical transport of carriers. Finally, significant energy barriers exist at interfaces between the Group III-V nitride materials and potential conducting substrates such as SiC, and between Group III-V nitride materials and all of the common metals which are needed for ohmic contacts in device applications. Accordingly, a low resistance ohmic contact is a fundamental problem for Group III-V nitride materials.